Writing the correct formula of the compound is a must in chemistry. Refer Writing formulae section
http://tutorsr.blogspot.com/2013/05/chemical-formulae.html
Stoichiometry is the ratio between the reactants and the products in a chemical reaction. When a chemical equation is balanced properly it is the number we put in front of each reactant and product. If a number is not written it indicates the number infront is 1
Mole concept is used because the atoms and compounds are having really very small weights and sizes. As a result it is not possible to measure them or tell the numbers by using the standard units of measurement.
